Oracle : buffer busy wait

Buffer Busy Waits usually happen on Oracle 10 and 11 mainly because of insert contention into tables or Indexes. There are a few other rare cases of contention on old style RBS segments, file headers blocks and freelists.

Before Oracle 10 and 11 there was one other major reason which was readers waiting for readers, ie one user does a phyiscal IO of a block into memory and a second user want to read that block. The second user waits until the IO is finished by the first user. Starting in 10g this wait has been given the name “read by other session“. Before Oracle 10g this was also a “buffer busy wait”.

The easiest way to analyse the bottleneck and find a solution is to use ASH (active session History) available in Oracle 10g with the diagnostics pack license or using Simulated ASH for free or using a product like DB Optimizer.

Data block class, which can be found in ASH, is the most important piece of information in analysing buffer busy waits. If we know the block class we can determine what kind of bottleneck:

If CLASS=

data block

IF OTYPE =

INDEX , then the insert index leaf block is probably hot, solutions are

Hash partition the index

Use reverse key index

TABLE, then insert block is hot,solutions

Use free lists

Put Object in ASSM tablespace

Segment header – If “segment header” occurs at the same time as CLASS= “data block” on the same object and the object is of OTYPE= “TABLE” then this is just a confirmation that the TABLE needs to use free lists or ASSM.

File Header Block – Most likely extent allocation problems, look at extent size on tablespace and increase the extent size to there are few extent allocations and less contention on the File Header Block.

free lists – Add free list groups to the object

undo header – Not enough UNDO segments, if using old RBS then switch to AUM

undo block – Hot spot in UNDO, application issue

How do we find the block class? With a quick query on the ASH data like:

If we find that CLASS=datablock, then we will want more information to diagnose, such as the object type “OTYPE” , object name and what kind of tablespace the object is stored in. The following query provides that information:

On version 8 and 9, the p3 value has a different meaning. Instead of meaning the block type (which is the best thing to know) it means the kind of buffer busy wait. There are only two values that matter to us, values in

100 range = read waits (basically just an IO wait)

Reader blocking Reader, ie one reader is reading a block in and another person wants to read this block and waits on a buffer busy wait p3=130.

200 range = write contetion (same as in 10g)

Writers blocking other writers for example while doing inserts either because of no free lists on the table or because everyone is inserting into the same index block.

If you have set up ASH style collection with S-ASH or have a product like DB Optimizer you can run a query like:

select

count(*) cnt,

o.object_name obj,

o.object_type otype,

ash.CURRENT_OBJ#,

ash.SQL_ID,

decode(substr(ash.p3,1,1),1,'read',2,'write',p3) p3

from v$active_session_history ash,

all_objects o

where event='buffer busy waits'

and o.object_id (+)= ash.CURRENT_OBJ#

group by o.object_name, o.object_type, ash.sql_id, ash.p3,ash.CURRENT_OBJ#

order by cnt

/

And see what kind of buffer busy waits there are and what the objects are:

CNT OBJ OTYPE CURRENT_OBJ# SQL_ID P3

--- ------- ------- ------------ ---------- ------

1 -1 1375352856 read

2 -1 996767823 read

2 -1 2855119862 write

17 -1 1375352856 write

89 TOTO1 TABLE 296030 1212617343 write

109 296022 1212617343 write

Often the Current_obj# is -1 so we can’t figure out what the object is . There is an alternative method

col block_type for a18

col objn for a25

col otype for a15

col event for a15

col blockn for 999999

col segment_name for a20

col partition_name for a15

col owner for a15

set timing on

/*

drop table myextents;

l

create table myextents as select * from dba_extents;

l

*/

select

count(*),

ext.owner,

ext.segment_name,

ext.partition_name,

ext.segment_type,

decode(substr(ash.p3,1,1),1,'read',2,'write',p3) p3

--ash.p1,

--ash.p2

from v$active_session_history ash,

myextents ext

where

event = 'buffer busy waits'

and ( current_obj# = -1 or current_obj#=0 or current_obj# is null )

--and sample_time > sysdate - &minutes/(60*24)

--and session_state='WAITING'

and ext.file_id(+)=ash.p1 and

ash.p2 between ext.block_id and ext.block_id + ext.blocks

group by

ext.owner,

ext.segment_name,

ext.partition_name,

ext.segment_type,

p3

--ash.p1,

--ash.p2,

--ash.sql_id

Order by count(*)

/

Because querying DBA_EXTENTS is a slow operation, I made a copy of DBA_EXTENTS which will be faster to query.

CNT OWNER SEGMENT_NAME PARTITION_NAME SEGMENT_TYPE P3

--- ------ -------------- --------------- ------------- --------

1 SYS _SYSSMU2$ TYPE2 UNDO read

1 SYS _SYSSMU3$ TYPE2 UNDO write

This second option of getting the object from P1 and P2 (file and block) should probably be done only with the users consent, because we would have to create a copy of the dba_extent table which might take a long time if it’s big.

No ASH ?

If you don’t have ASH data you will have to do some guess work.

Block Class (block type)

The first step in finding out the source of buffer busy waits is looking at

V$waitstats

This will tell us what kind of datablocks we have contention on.

File with contention

You can also get an idea of what file contains the object with the buffer busy waits by looking at:

X$KCBFWAIT

Object with contention

Starting in version 9i there is the table

v$segstat

That will list the objects with buffer busy waits.

If you are on version 7 or 8 good luck finding the object without setting up ASH style data collection.

Why do buffer busy waits happen?

To put it most succinctly, buffer busy waits happen because two users want to change a block at the same time. Two users can change the same block, or even same row “at the same time” ie without committing, but that’s different from the actual operation of modifying the block. The modification on the block in RAM, or computer memory, can only be done by one process at at time in order to avoid memory corruptions. Different users can modify different blocks at the same time but only one user or process can modify a the same block at a time.

In order to really understand what’s going on we have to take a look at how Oracle manages memory and block access and modifications.

Here is the layout of

Above is a diagram shows some of the essential parts of Oracle in regards to performance tuning.

In the machine memory are

Oracle’s SGA, or System Global Area, a memory that is shared between Oracle users

LGWR – log writer process

DBWR – database writer process

User1,2,3 … – user processes, in this case “shadow processes”

On the machine file system are

Redo log files

Data files

The SGA is composed of (among other things)

Log Buffer

Library Cache

Buffer Cache

What’s important for understanding buffer busy waits is how the buffer cache is managed. Here is view of the buffer cache with more components:

In order to access a block, a user (shadow process) has to get a latch (cache buffer chains latch) which protects buckets or linked lists of buffer headers. Once the header desired if found the latch is released. The buffer headers point to the actual data block in memory. Before modifying a block in memory a user has to lock the buffer header. The buffer header is locked any time a modification is made whether it is reading a block into memory or modifying a block that is already in memory. Usually the header is locked only for a brief amount of time but when there is a lot of concurrent access the buffer header can become a bottleneck.

BBW when readling data – read by other session

A buffer busy can happen on oracle 7,8 and 9 when one user is reading a block into memory and a second user wants to read that block. Instead of the second user trying to read that block into memory as well, they just wait for the first user to finish. Starting in Oracle 10, this kind of wait was renames “read by other session”

BBW on insert

If multiple concurrent users are inserting into a table that doesn’t have free lists or is not in an ASSM tablespace then all users will end up inserting into the same block, the first one on the free list and this block will become the hot block

by adding free lists or moving the table to an ASSM tablespace we will alleviate the bottleneck.

Multiple free lists:

The other option is ASSM or Automatic Segment Space Management which is set at the tablespace level.

In this case free block information is kept in Level 1 BMB (or bitmapped blocks). These Level 1 BMBs are chosen by a hash on the users process ID thus distributing the inserts across the table.

The inserts would look something like this (somewhat exaggerated drawing)

the ASSM BMB blocks take up more space in the table , about 1 extra block for every 16 data blocks and there is overhead first looking in the header/level 3 BMB block then going to the Level 2 then level 1 and finally to the datablock but all in all ASSM is worth reduced costs of management verses free lists.

Identifying and creating ASSM tablespaces

Which tablespaces are ASSM or not?

select

tablespace_name,

extent_management LOCAL,

allocation_type EXTENTS,

segment_space_management ASSM,

initial_extent

from dba_tablespaces

TABLESPACE_NAME LOCAL EXTENTS ASSM

--------------- ---------- --------- ------

SYSTEM LOCAL SYSTEM MANUAL

UNDOTBS1 LOCAL SYSTEM MANUAL

SYSAUX LOCAL SYSTEM AUTO

TEMP LOCAL UNIFORM MANUAL

USERS LOCAL SYSTEM AUTO

EXAMPLE LOCAL SYSTEM AUTO

DATA LOCAL SYSTEM MANUAL

creating an ASSM tablespace:

create tablespace data2

datafile '/d3/kyle/data2_01.dbf'

size 200M

segment space management auto;

BBW on index (because of insert)

If users are inserting data that has a rising key value, especially a monotonically rising value, then all the new inserts will have to update the leading edge leaf block of the index and with high concurrent inserts this can cause buffer busy waits.

Solutions

Hash partition the index

Reverse Key Index

BBW on old style RBS

IF block class > 18 it’s an old style RBS segment

Select CURRENT_OBJ#||' '||o.object_name objn,

o.object_type otype,

CURRENT_FILE# filen,

CURRENT_BLOCK# blockn,

ash.SQL_ID,

w.class ||' '||to_char(ash.p3) block_type

from v$active_session_history ash,

(select rownum class#, class from v$waitstat ) w,

all_objects o

where event='buffer busy waits'

and w.class#(+)=ash.p3

and o.object_id (+)= ash.CURRENT_OBJ#

Order by sample_time;

OBJN OTYPE FILEN BLOCKN SQL_ID BLOCK_TYPE

----------- ------ ------ ------ ------------- ------------

54962 TOTO1 TABLE 16 45012 8gz51m9hg5yuf data block

54962 TOTO1 TABLE 16 161 8gz51m9hg5yuf segment header

0 14 9 8gz51m9hg5yuf 87

0 14 9 8gz51m9hg5yuf 87

IF the block is of class > 18, the there will be no object name, so we have to look it up ourselves to be sure:

select segment_name,

segment_type

from dba_extents

where

&P2 between

block_id and block_id + blocks – 1

and

file_id = &P1 ;

Plug in 14 for P1 the file # and 9 for P2 the block number:

SEGMENT_NAME SEGMENT_TYPE

-------------- --------------

R2 ROLLBACK

solution

move to new AUM or Automatic Undo Mangement

alter system set undo_management=auto scope=spfile;

BBW on a file header

The ASH data has two different fields that indicate the file # and block # when the wait is a buffer busy wait.

For a buffer busy wait

File # = p1 *and* File # = current_file#

Block # = P2 *and* Block # = current_block#

if p1 != current_file# or p2 != current_block# then use p1 and p2. They are more reliable.

If you are getting buffer busy waits on the file header block for a tempfile (datafile in a temporary tablespace) then try increasing the “next extent” size in the temporary tablespace.

This wait can happen when lots of extents are being allocated in the temporary tablespace.

What Would ADDM do?

Interstingly enough the ADDM page doesn’t show the new load that has recently come on the system but the analysis is there. I clicked on the next to bottom line in the page, “Read and write contention on database blocks was consuming significant database time.

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Comments

Interesting link sent to me by Yong Huang http://www.askmaclean.com/archives/gc-buffer-busy-log-flush-sync-log-file-sync.html
which has an ADDM which on the particular instance in question, gave the following interesting analysis :
RECOMMENDATION 2: Host Configuration, 12% benefit (507182 seconds)
ACTION: Investigate the possibility of improving the performance of I/O
to the online redo log files.
RATIONALE: The average size of writes to the online redo log files was
40 K and the average time per write was 10 milliseconds.
ADDITIONAL INFORMATION:
Waits on event “log file sync” were the cause of significant database
wait on “gc buffer busy” when releasing a data block. Waits on event
“log file sync” in this instance can cause global cache contention on
remote instances.